In the models proposed by Jens Wegner, the plane of epidermal division is regulated by the force exerted upon it by the proliferation of underlying cells. In this model it is tempting to envisage the epidermis as a type of "bag" which is stretched over underlying cells and exerts an inward pressure on them, thus restricting their growth. The switch from anticlinal to periclinal cell divisions observed in the meristematic L2 after L1 removal in the experiments of Reinhardt et al. appears to support this view. Interestingly, similar regular periclinal divisions are induced when pressure is applied to Coleus internodes and removed one day later (Lintilhac 1981), and in underlying parenchyma when epidermal cells are excised from epicotyl in Citrus (Bruck 1985b). This could be interpreted, then, as a universal reaction to release of pressure, but it could equally well reflect a universal response to wounding, and the two possibilities are very difficult to separate.
Another observation which supports a restrictive role for the epidermis in organ growth is that, at least at the early seedling stage, mutants where epidermal integrity is compromised often appear bigger, and grow faster than wild-type plants in tissue culture conditions where humidity is very high (Ingram, unpublished results). Whether this apparent increased growth is due to increased cell division, or simply to abnormal cell expansion has not been accurately ascertained.
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